1. Field of the Invention
This invention pertains to a heat transfer device of the general type referred to as a heat pipe and comprising an elongated sealed tube or pipe containing a working fluid which is substantially continuously evaporated, transported and condensed to transfer heat.
2. Background
In the art of heat transfer devices, the heat pipe has found substantial commercial application. Heat pipes are particularly advantageous for applications where heat must be transferred between a source, usually a fluid, to a sink, also usually a fluid, and wherein the fluids or other forms of source and sink cannot be mixed or even brought into close proximity to one another. Heat pipes are also advantageous because of their structural simplicity and their heat transfer capacity as compared with their physical bulk.
However, prior art types of heat pipes have certain limitations with regard to their application in situations where the evaporator section of the heat pipe is required to be very small but also be capable of transferring a relatively high rate of energy away from a source of heat such as certain types of electronic equipment or other equipment wherein more conventional heat transfer apparatus cannot be used. Prior art heat pipes also do not function reliably in applications wherein the evaporator section must be maintained in a generally horizontal configuration and the condenser section of the heat pipe is required to be elevated with respect to the evaporator section. Certain other problems associated with heat pipes include that of starting the flow of fluid in the desired flow path within the heat pipe during operational start-up of the heat pipe itself.
Early developments in the art of heat pipes are traced to the devices described in British Patent specification No. 22,272, accepted Dec. 2, 1893, to Perkins et al. The Perkins et al patent describes various configurations of elongated closed-end tubes or pipes adapted for transferring heat between a source and a sink by the evaporation of a fluid medium such as water in an evaporator section of the tube and transmission of the water vapor to the opposite end of the tube which is exposed to a source of fluid for cooling and condensing the vapor within the tube and whereby it is intended that the condensed fluid flow back to the evaporator section in a substantially continuous operating cycle.
It has been found that the teachings of the Perkins reference are severely limited with respect to the concept of a closed-end heat pipe utilizing water as a working fluid. For example, Perkins fails to suggest how much working fluid in liquid form should be used with the various embodiments described in the reference, suggests that the working position of the pipes never be horizontal, and suggests that water cannot be used with a tube or pipe having less than 0.50 inches inside diameter. Most of the tubes described in the Perkins reference are not provided with any internal structure, such as wicking or flow separator devices, with the exception of an embodiment having a central web extending substantially throughout the length of the pipe and an embodiment having a so-called tube within a tube.
The disadvantages of the Perkins tubes, including the embodiment of a tube within a tube, have been pointed out in my prior U.S. Pat. No. 4,020,898 (herein referred to sometimes as the '898 patent), issued May 3, 1977, and assigned to the assignee of the present invention, Q-dot Corporation, Garland, Tex. The '898 patent is directed to an improvement in heat pipes having relatively long or large area evaporator and condenser sections wherein a so-called flow separator tube is provided within the envelope of the heat pipe itself for conducting the condensed liquid from the condenser section back to the evaporator section so as to avoid the problems connected with single tube heat pipes without flow separating structure. The tube within a tube type of heat pipe disclosed and claimed in the '898 patent holds certain advantages over heat pipes which are provided with capillary wick structures, as the latter are difficult and expensive to install properly within the outer envelope tube of a heat pipe.
Although the configuration of the heat pipe described in the '898 patent overcomes certain problems associated with the simple heat pipe, or Perkins tube, and heat pipes with capillary wick structures, it was previously believed that the flow of working fluid vapor from the evaporator section to the condenser section must take place in the space between the inner wall surface of the outer tube or envelope and the outer wall surface of the flow separator or inner tube. In fact, a substantial body of patent literature and many years of testing and development by this art worker have indicated that the so-called conventional mode of circulation of working fluid was necessary for reliable operation of heat pipes.
It was determined in accordance with the '898 patent that the flow separator or liquid return tube should have an inside diameter equal to approximately 30% to 40% of the inside diameter of the outer tube, the liquid return tube should be between 65% and about 85% of the length of the outer tube, and the working fluid, in the liquid phase at working temperatures, should fill about 50% and 75% of the volume of the outer tube. Moreover, the heat pipe disclosed in the '898 patent has relatively long evaporator and condenser sections, which may, in fact, extend toward a point almost contiguous with each other and be separated by a wall or partition.
The improvements described and claimed in U.S. Pat. No. 4,020,898 show substantial superiority over the Perkins tubes with flow separator structure. However, in commercial applications the improved heat pipe described in the '898 patent is generally limited to a substantial straight pipe, is limited to a minimum diameter of 0.50 to 0.75 inches, and performs best when the evaporator section is inclined downward with respect to the horizontal.
As a result of the experiments with the Perkins tube described in the '898 patent, it was not believed that a heat pipe could be satisfactorily developed which has a horizontally disposed, or near horozintally disposed, and relatively small area, or short, evaporator section and which also could utilize a generally tubular flow separator of the type previously developed. This is particularly true for relatively long heat pipes and those arranged wherein an intermediate or so called adiabatic section and the condenser section must be inclined upward with respect to the evaporator section. It has also been believed that heat pipes with relatively short evaporator and condenser sections on the order of 10% and 25% to 30%, respectively, of the overall tube length could not be reliably started in a conventional or reverse mode of fluid flow between the respective evaporator and condenser sections.
Moreover, in accordance with prior art developments, it was believed that the dimensional parameters for the flow separator or liquid return tube should adhere to the limits described in the '898 patent. The prior art configurations of heat pipes have also been characterized by the requirement for relatively long evaporator sections in order to assure reliable starting and operation since localized or concentrated applications of heat to small areas of the evaporator section tend to form pockets of vapor which block the counterdirectional flow of the working fluid. This has prevented application of heat pipes to situations wherein very concentrated sources of heat are generated in small spaces and at high rates, such as in various types of electronic equipment previously mentioned.
However, with the need to develop a heat pipe having a generally horizontal, short evaporator section and with the need to develop heat pipes having a wide range of lengths (10 to 150 inches) and relatively small diameters (less than 0.50 inches), it has been determined that a heat pipe operating in a mode substantially the reverse of that previously known and suggested by the prior art is capable of reliable starting under a wide variety of heat rate or power applications, has a heat transfer rate capacity equal to or superior to other known types of heat pipes, has coefficients of heat transfer for the respective evaporator and condenser sections greater than heat pipes operating in the so-called normal mode and is particularly adapted for use with concentrated sources of heat applied to a small area of the evaporator section. For purposes of description herein, the so-called normal or conventional mode of operation is that wherein, with a tubular-type flow separator or fluid conduit within the envelope of the outer tube, vapor flows from the evaporator section to the condenser section in the annulus formed between the inner and outer tubes and the inner tube is adapted to carry liquid from the condenser section to the evaporator section. The heat pipe of the present invention is adapted for operation in the so-called reverse mode wherein a tubular conduit is provided within the envelope of the outer tube of the heat pipe and which is operable to carry working fluid vapor from the evaporator section to the condenser section. The condensed liquid working fluid is returned to the evaporator section in the generally annular passage formed between the inner wall of the outer tube and the outer wall of the inner tube.